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Flow Measurement

Orifice Drain and Vent Hole

What is the Purpose of Orifice Drain Hole?

Drain hole is a small hole which is provided in the lower region of orifice plate. Drain hole is required in gas flow service where liquid entrapment may occur.

Drain hole size could affect the accuracy of flow measurement. However, if the diameter of the drain hole is less than 10% of the orifice bore, then the unmeasured flow is less than 1% of the total flow.

Drain hole is not recommended in dirty fluid service or slurries as the hole could be plugged. In this application, the use of eccentric orifice plate becomes alternative.

orifice plate drain hole

What is the Purpose of Orifice Vent Hole?

Vent hole is a small hole which is provided in the upper region of orifice plate. Vent hole is required in liquid flow service where gas entrainment may occur.

Vent hole size could affect the accuracy of flow measurement. However, if the diameter of the vent hole is less than 10% of the orifice bore, then the unmeasured flow is less than 1% of the total flow.

Vent hole is not recommended in dirty fluid service or slurries as the hole could be plugged. In this application, the use of eccentric orifice plate becomes alternative.

orifice plate vent hole

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1 comment

D.N.Sharma May 14, 2018 at 11:21 pm

Your statement regarding decrease in accuracy due to drain and vent hole is not correct.There is no change in accuracy as it is taken care during orrifice design

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